1. Field of the Invention
The present invention relates to an X-ray diagnostic apparatus of the type having an X-ray source that emits a diverging X-ray beam bundle that strikes a planar X-ray image converter at various angles, said converter having a substrate, with image elements on the substrate arranged in a matrix with a semiconductor layer and a layer that absorbs X-rays.
2. Description of the Prior Art
An X-ray diagnostic apparatus of the above general type is known, for example, from U.S. Pat. No. 5,528,043.
In many applications of such X-ray detectors, the X-rays do not strike the X-ray-sensitive surface of the X-ray detector perpendicularly, but rather obliquely. In methods employing projection geometry, for example radiography and transillumination, this leads to a worsening of the spatial resolution.
This effect is more pronounced
1) the closer the X-ray tube is to the X-ray detector, PA1 2) the larger the lateral dimensions of the X-ray detector are, PA1 3) the thicker the radiation-absorbing layer is, and PA1 4) the finer the image point raster is in a digital image detector.
Mammography presents a particularly disadvantageous case. Here the distance from the X-ray focus to the X-ray detector or, respectively, film is typically only approximately 60 cm, in contrast to the standard distance of approximately 100 cm in radiography. Since the X-ray tube is arranged directly over the breast (chest) wall, the lateral extension from the striking point of the normal ray, calculated in the direction of the tip of the breast, amounts to the full edge length of the X-ray detector, while in radiography the normal ray strikes in the center of the detector, and thus only half the edge length must be taken into account. The image point raster is likewise very small, due to the required high spatial resolution. It typically has a value of &lt;100 .mu.m.
In order to achieve a high quantum absorption, the layer that absorbs the X-rays must be constructed as thick as possible, which results in the disturbing effect being further increased.
FIG. 1 shows the spatial resolution in the form of a simulated curve of the modulation transfer function (MTF curve). In FIG. 1, the contrast is plotted over the spatial frequency. The group of curves shows the course of the MTF in 4-cm steps from one locus directly underneath the X-ray tube (upper curve) up to a locus at a distance of up to 24 cm--the lowest curve corresponds to the MTF at a distance of 24 cm. A focus distance of 57 cm, a thickness of the absorbing layer of 200 .mu.m and an image point raster of 85 .mu.m are assumed. The MTF was approximated using the sinc function. It can be seen that the MTF clearly decreases toward the edge of the image. In the book "Das Rontgenfernsehen" by Gebauer et al., 2.sup.nd ed., Georg Thieme Verlag, Stuttgart, it is stated on pages 26ff that the threshold of recognizability is located at a spatial frequency at which the contrast has decreased to a value of 0.05 (see in particular page 28, right column, penultimate paragraph). This threshold value was plotted as a straight line in FIG. 1. It can thus be seen from FIG. 1 that, given perpendicular incidence, the resolution capacity of a spatial frequency of &gt;10 lp/mm (line pairs per mm) decreases to as low as 4 at a distance of 24 cm.
This estimation of the MTF is, however, pessimistic, since at layer thicknesses that ensure a very high quantum efficiency (DQE), the absorption lengths of the X-ray radiation are smaller than the layer thickness. With selenium and an average quantum energy of 22 keV, for example, an average absorption length of 65 .mu.m results.
From U.S. Pat. No. 5,570,403, a computed tomography apparatus is known that has various scintillator thicknesses.
In Japanese Application A 61-201 183, a scintillator is specified that is fashioned thinner in the middle detector region than in the outer detector region.